This invention relates to a process for the production of polyurethane plastics in which N-hydroxyalkylsubstituted cyclic compounds containing amidine groups are used as catalysts.
Many amines are effective catalysts for addition reactions of polyols with isocyanates (cf. K. C. Frisch, L. P. Rennao, "Catalysis in Isocyanate Reactions", in J. Macromol. Sci. --Revs. Macromol. Chem. Soc C 5 (1), 103-150). The most active amines which generally contain tertiary nitrogen (for example, 1,4-diazabicyclo-(2,2,2)-octane), must be used in amounts of from 0.04 to 0.5 parts by weight (based on the polyol used). Other less reactive amines must be used in considerably larger quantities.
Even small quantities of amine catalysts, however, give rise to serious disadvantages. Since the amines are generally tertiary amines which cannot be incorporated in the polyurethane product, a noxious odor persists for some time after production. Such an odor is particularly undesirable when everyday articles such as upholstery materials, fittings for motor vehicles, shoes, furniture, etc. are being made. These amines are also frequently responsible for the yellowing of light-colored leather or plastics surfaces.
Another disadvantage of amine catalysts which are not incorporated in the product polyurethane is that they may exude or be transported to the surface by other physical and/or chemical processes. For example, a polyurethane material catalyzed with diazabicycloundecene of the type used for the production of steering wheels and headrests, may become covered by a white coating in which diazabicycloundecene is found. Similarly, in cases where triethylene diamine (Dabco.sup.(R)) is used as catalyst in the production of polyurethane shoe soles, discoloration is frequently observed to affect light-colored upper leathers, making it impossible to foam the sole directly onto the upper. This discoloration is caused by triethylene diamine escaping during the foaming process. A similar effect is observed in cases where non-incorporable tetrahydropyrimidines are used as catalysts.
German Offenlegungsschrift No. 1,950,262 describes amidines as catalysts in the production of polyurethanes from aliphatic isocyanates. German Offenlegungsschrift No. 2,737,671 describes the use of cyclic amidines having tetrahydropyrimidine or imidazoline structures as catalysts for the production of polyurethanes. Bicyclic amidines as polyurethane catalysts are described in German Offenlegungsschrift No. 1,745,418.
German Offenlegungsschrift No. 2,601,082 describes the production of polyurethanes using incorporable catalysts of the amidine type. However, the aminopyridines disclosed therein show sufficient activity only for the production of polyurethanes from aromatic polyisocyanates. The catalytic activity of these aminopyridines is inadequate for the production of polyurethanes from aliphatic polyisocyanates. Therefore, the aminopyridines previously used as catalysts in the production of polyurethanes have often been inadequate for practical applications.